In the context of spacecraft, attitude control is control of the angular position and rotation of the spacecraft, either relative to the object that it is orbiting, or relative to the celestial sphere. A spacecraft is a vessel, craft or device designed to operate beyond the surface of the Earth in outer space. ... The celestial sphere is divided by the celestial equator. ...

In flight dynamics, the orientation is often described using three angles called yaw, pitch, and roll. Flight dynamics is the study of orientation of air and space vehicles and how to control the critical flight parameters, typically named pitch, roll and yaw. ...

Sensors

Inertial guidance system 

The most conventional sensor. It tracks the current position and angle based on earlier position, and acceleration. Measurement of angular velocity is also critical, for attitude control systems that simulate frictional dampening to prevent oscillating corrections.

Orbital Gyrocompassing 

Typically combined with an infrared horizon, a gyrocompass finds the perpendicular to the plane of the orbit and provides a yaw indicator.

Sun sensor 

Another common sensor. This can be as simple as some solar cells and shades, or as complex as a steerable telescope.

Star tracker 

Another common sensor. This is usually a small (1 inch) telescope mounted at a fixed angle to a rotating bearing. A photocell or solid-state camera arrangement sees the star. There are 57 bright navigatonal stars in common use. One of the most commonly used is Sirius (the brightest). However, for more complex missions entire starfield databases are used to identify position.

Earth horizon indicator 

This instrument can detect certain wavelengths of emitted light from the 'limb' of the Earth's atmosphere just at the horizon. It can be a scanning or a staring instrument. Infrared is often used, which can function even on the dark side of the Earth.

An inertial guidance system consists of an Inertial Measurement Unit (IMU) combined with control mechanisms, allowing the path of a vehicle to be controlled according to the position determined by the inertial navigation system. ... Sirios (α CMa / α Canis Majoris / Alpha Canis Majoris) is the brightest star in the night-time sky, with a visual apparent magnitude of −1. ... An ephemeris (plural: ephemerides) (from the Greek word ephemeros = daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, the planets, asteroids or comets in the...

Algorithms

Algorithms are the computer programs that receive input from the sensors and calculate the appropriate acutator commands. These commands, whether for reaction wheels, jets, or other acuators, are intended to apply forces and torques to rotate or translate the spacecraft to the desired attitude. The attitude data which is processed by the algorithms comes from the attitude sensors, for example gyroscopes, sun sensors, or star trackers. The algorithm can be a very simple proportional control or a complex nonlinear estimator or many in-between types, depending on the mission requirements. Typically, the attitude control algorithms are part of the spacecraft flight software running on the hardware of the command and data handling equipment. Flowcharts are often used to represent algorithms. ... A computer program is a collection of instructions that describe a task, or set of tasks, to be carried out by a computer. ... In physics, force is an influence that may cause a body to accelerate. ... Torque applied via an adjustable end wrench Relationship between force, torque, and momentum vectors in a rotating system In physics, torque can informally be thought of as rotational force or angular force which causes a change in rotational motion. ... attention A proportional control system is a type of linear feedback control system. ... Software, or program, enables a computer to perform specific tasks, as opposed to the physical components of the system (hardware). ... Computer hardware is the physical part of a computer, including the digital circuitry, as distinguished from the computer software that executes within the hardware. ...

Actuators

Attitude control can be maintained by

Thrusters 

The conventional, low-risk solution is to use thrusters (usually monopropellant rockets), organized in a Reaction control system. However, they use fuel. In normal station-keeping, the fuel-efficiency of an attitude control system is determined by the smallest time it can thrust. In practice, controllers successfully adjust down to this amount, and then the spacecraft oscillates slightly, with a tiny blip of thrust in one direction, and a few tens of seconds later, an opposing blip of thrust. To conquer this fuel limitation on mission lengths and masses, sometimes auxiliary attitude control systems are used that can resolve lower angular velocities, notably momentum wheels and solar sails, or smaller, lower thrust vernier thrusters. The Soviet Luna 3 spacecraft was the first 3-axis stabilized spacecraft to employ an attitude control system with inertial guidance and compressed nitrogen microthrusters.

Momentum wheels 

These are electric motors that spin in the direction opposite to the direction the spacecraft needs to turn. Since they're computer controlled, and usually a small fraction of the spacecraft's mass, they give precise control. The biggest problem is that they move, and therefore their bearings can fail. To increase lifetimes, they can have magnetic bearings. To maintain orientation in three dimensional space a minimum of three must be used with additional units providing redundancy.

Control moment gyros 

These are momentum wheels that are mounted on a gyroscopic frame to allow control and rotation of the spacecraft. An advantage of a CMG over a momentum wheel is that since it allows control on multiple axes it reduces the number of units needed. By reducing the number of units needed, an overall reduction of cost and mass is achieved even though a CMG is a bit more expensive in terms of both cost and mass. The maximum torque than can be exerted by a CMG is also greater than for a momentum wheel, making it better suited to large spacecraft. The negative to a CMG is the additional complexity increases the possible points of failure. The International Space Station uses a set of four CMGs mounted above the Destiny laboratory in the Z1 truss unit.

Precession 

Two counter-rotating gyroscopes have net zero momentum. If their axes point in the same direction, then pulling two nearby bearings together will cause both gyroscopes to precess in the same direction, rotating the spacecraft. Pushing the bearings apart causes rotation in the opposite direction. Both gyroscopes can also be used as momentum wheels to roll the spacecraft, permitting full-authority attitude control with only two wheels and minimal (or no) use of attitude jets.

Solar sails 

Small solar sails and thermal radiation adjustment devices (adjustable reflectors) have been successfully used (e.g. on Pioneer 10) to make small attitude control and velocity adjustments. This can save large amounts of fuel on a long-duration mission by damping the oscillation rates more precisely than thrusters can achieve.

Mass distribution or Gravity Gradient 

A great virtue of this system is that it requires no control system. This is how the earliest satellites were stabilized. In orbit, a long spacecraft will spontaneously orient so that its long axis points at the planet's center of mass. This is caused by a tidal force. The upper part is moving faster than orbital speed, and pulls away. The lower part is moving slower than orbital speed and pulls down. Sometimes tethers are used to connect two parts of a satellite, spread by a tide. A problem with simple tethers is that meteroroids as small as a grain of sand can cut them.

Spin Stabilization 

For a mid-range of attitude control accuracy, the entire satellite itself can spin on a particular axis to maintain its orientation. This creates an axis of momentum which will maintain its orientation despite small perturbations. Precision control can then be enhanced with other actuators. This is only applicable to certain missions with a primary axis of orientation that does not need to change dramatically over lifetime of the satellite and no need for extremely high precision pointing. It is also useful for missions with instruments that must scan the starfield or the Earth's surface or atmosphere.

Magnetic field coils (also known as Magnetic Torque coils) or (on very small satellites) permanent magnets 

These exert a moment against the local magnetic field. This only works where a magnetic field exists. One classic field coil is actually a conductive tether in a planetary magnetic field.

Pure passive attitude control 

For low resolution attitude control in an Earth orbit, tidal and magnetic pointing can be combined with viscous damping to get a completely passive system. Simple uncontrolled magnets and tidal pointing have a limited pointing accuracy, because the spacecraft can oscillate around energy minima. A simple solution to this oscillation is a viscous damper, a small can or tank of fluid mounted in the spacecraft, possibly with internal baffles to increase internal friction. Fast oscillation of the satellite will be turned into heat within the viscous damper. Slow oscillations have less effect. Where the satellite needs to have a preferred end (like a camera) point at the planet, usually several small electrically fired, solid-fuel rockets are mounted on one end, each with thrust enough to flip the satellite end-for-end.

A thruster is a small propulsive device used by spacecraft and watercraft for station keeping, attitude control, or long duration low thrust acceleration. ... A monopropellant rocket (or monoprop rocket) is a rocket that uses a single chemical as its power source and propellant. ... A reaction control system (abbreviated RCS) is a subsystem of a spacecraft. ... The Vernier Thruster is a special case of the general type of spacecraft attitude control thruster, which is a smaller thrust motor than main attitude control motors and is used for fine adjustments to the attitude or velocity of a spacecraft. ... Luna 3, an automatic interplanetary station of the Luna program, was the third spacecraft successfully launched to the Moon and the first to return images of the lunar far side. ... Gyroscope of the International Space Station A momentum wheel or reaction wheel is a type of flywheel used primarily by spacecraft to change their angular momentum without using fuel for rockets or other reaction devices. ... Rotating magnetic field as a sum of magnetic vectors from 3 phase coils An electric motor converts electrical energy into kinetic energy. ... A magnetic bearing is a bearing which supports a load using magnetic levitation. ... Control Moment Gyro(scope) is an attitude control device generally used in satellite attitude control systems. ... ISS redirects here. ... Precession refers to a change in the direction of the axis of a rotating object. ... Concept image of a solar sail spacecraft in the process of unfurling sails. ... Pioneer 10 in the final stage of construction Pioneer 10 was the first spacecraft to travel through the asteroid belt, and was the first spacecraft to make direct observations of Jupiter. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ... Artists conception of satellite with a tether Tether propulsion uses long, strong strings (known as tethers) to change the orbits of spacecraft. ... To meet Wikipedias quality standards, this article may require cleanup. ...

See also

• Aircraft attitude

Aircraft attitude is used to mean two closely related aspects of the situation of an aircraft in flight. ...

External link

• Attitude Control Example With A Dynamically Astable Platform